In a power plant environment, it is often necessary to adjust process flows of a fluid to lower temperatures by means of a cooling device. Particularly in the case of gas turbine applications, such cooling devices are necessary in order to bring bleed air taken from the compressor, for example, to a lower temperature level. The cooled bleed air can be used, in turn, for purging operations or for the thermal insulation of the casing of the gas turbine. However, since relatively large quantities of thermal energy arise in gas turbines, especially in stationary gas turbines, the cooling devices provided must be able to allow effective cooling of the respective process medium.
Typical cooling devices used in a power plant environment have a plurality of controllable fans, which can each produce an air flow with a specified flow direction, wherein, after thermal interaction with a heat exchanger, this air flow can remove heat therefrom as a secondary medium for cooling purposes. If it is necessary to provide a constant temperature in the primary medium to be cooled in the heat exchanger, the fans are typically operated in a constant manner above a certain minimum speed, wherein the flow of the primary medium is likewise set to a constant level.
When a plurality of controllable fans that thermally interact with the same heat exchanger is provided, however, there can be backflows from one fan to another fan, and therefore the controllable fans are generally closed by a closing flap, in particular to the extent that the fans are no longer needed. Closing flaps of this kind are typically opened by the dynamic pressure of an air flow from a fan, wherein, after the specified minimum speed has been reached, the closing flap can be held open by the flow pressure. Normally, each fan is assigned a closing flap of this kind, wherein the closing flaps perform an opening movement in the region of the minimum speed. However, closing flaps of this kind can not only perform a brief uncontrolled movement in the limiting pressure range but can also strike against parts of the casing or of the mounting, and this should be avoided.
As part of increasing the flexibility of power plants, it is furthermore also necessary not to continuously cool process flows of a primary medium over a prolonged period, or to vary the cooling capacity. Thus it may be necessary, for example, also to be able to provide the cooling capacity at intermittent times. To this extent, there are frequent on and off switching operations of the fans, as a result of which high loads are imposed on the fan motors and, as a consequence, the service life of a fan is impaired. Moreover, the differences in the way in which the individual fans are operated result not only in a reduction in service life but also in increased noise pollution as well as largely uncontrolled radiation of heat by the motors of the fans.
In the context of increasing flexibility of power plant operation, it is furthermore found to be necessary not only that it should be possible to operate fans continuously above the specified minimum speed but also that operation below the specified minimum speed should be achieved. However, secure operation of the closing flaps can no longer be achieved in consequence of underspeed operation, and therefore this circumstance too requires frequent on and off switching operations of the fans.
In addition to the already mentioned disadvantages of unfavorable heat generation and also increased noise pollution due to flexible operating behavior, it is found that the temperature of the primary media too can no longer be adjusted to a constant value owing to the operating restrictions imposed by the flaps. On the contrary, the initial temperature of the primary medium emerging from the heat exchanger fluctuates so widely that it can impair operation of the power plant.